Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 232220
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
315 PM PDT Sat Jun 23 2018

.SYNOPSIS...An offshore upper ridge will move into Western
Washington bringing clearing skies and dry conditions this afternoon
and tonight with sunny and warmer weather on Sunday. An upper trough
will arrive Monday for a chance of showers and cooler weather.
Tuesday and Wednesday will be partly to mostly sunny, then another
upper trough will be over the area late in the week.

&&

.SHORT TERM...Clouds continue to thin out slowly but surely over the
CWA this afternoon as upper level ridging spreads in from the east.
Temps may fall a little short of forecasted highs today...but
lowland locations still likely to see highs get into the low
70s...so not too far off. Nothing of significance on current radar
imagery.

The aforementioned upper level ridge will continue to build over the
area this afternoon and this evening. Models remain consistent on
the progressive nature of this ridge...as it will make its way out
of the area overnight tonight...although it is a little hesitant to
leave WA entirely as it lingers over the eastern half of the state
into early Sunday afternoon. This will keep conditions over W WA
sunny and dry for Sunday...with afternoon highs expected to reach
near 80 in the interior lowlands.

A front in advance of an upper level low will reach the coast by
Sunday evening and moving inland overnight...with POPs area-wide
Monday morning. Models still generally in agreement that the upper
low will lift northeastward through the day with dry conditions re-
emerging over the southern half of the CWA by late Monday afternoon.
This will still manage to knock temps down...with highs expected to
be in the upper 60s to around 70 in the interior. Models do disagree
a bit on the timing of the exit of this system...the ECMWF wants it
to linger just a bit into Monday night...but both models do agree on
an upper level ridge starting to make its presence known by
Tuesday...warming temps and bringing a return to dry conditions.  SMR

.LONG TERM...This ridge looks to fizzle out by the time it makes it
to the coast...leaving zonal flow over the area for Wed making for a
dry day and keeping temps pretty level with highs right around 70 in
the lowlands. Another upper level trough will enter the area
afterward bringing POPs back into the forecast for both Thursday and
Friday. Both models in agreement with regards to a Pacific high
pressure system likely to be the next influence on W WA
weather...but there is some disagreement on how soon or even if
moisture will move out...the GFS opting for a drier solution while
the ECMWF allows POPs to linger. Until a consensus emerges...opted
to split the difference.  SMR

&&

.AVIATION...Flow aloft tonight will be westerly, then will become
more southwesterly Sunday as a weak front approaches the coast.
Moderate surface onshore flow will continue tonight, weaken Sunday,
then increase again Sunday night. The air mass is stable with marine
stratus currently dissipating over most of the area.

Most terminals have ceilings around 3k ft or no ceiling. Improving
trend will continue the rest of the afternoon until all clouds have
scattered out. Mostly clear skies are likely through the night into
Sunday. Some stratus will reform early Sunday morning, mainly
western sections, but there could be a few hours of MVFR ceilings
around Puget Sound. Have indicated 1k ft scattered deck Sunday
morning in most TAFs for now.

KSEA...Scattering by 23Z seems like a good bet. Onshore gradients
are considerably weaker late tonight and Sunday so there is a decent
chance of no ceiling Sunday morning. Sunny skies most of Sunday.
Northerly wind 5-10 kt. CHB

&&

.MARINE...Onshore flow will continue into next week with westerlies
in the Strait of Juan de Fuca rising to at least 20-30 kt each
evening, but possibly reaching gale force for a few hours. Tonight
is one of the 20-30 kt nights.

A cold front will reach Western Washington Sunday night ahead of the
next upper trough. Currently Sunday evening looks like the best bet
for gale force westerlies in the strait. Have put 25-35 kt in the
forecast and issued a gale watch for Sunday night. Wednesday evening
could also have stronger than usual onshore gradients. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from Sunday afternoon through late Sunday night for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 5 PM PDT Sunday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$

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